Compressor Suction Scrubber Demister Selection

A compressor suction scrubber demister protects rotating equipment. The purchase decision should therefore be judged by carryover risk, not by pad price alone.

Translate the consequence into a design basis

The process team should state what liquid can reach the compressor, how much routine and upset loading is credible, and what downstream symptom is unacceptable. A free-liquid trip, lubricant contamination, erosion or reduced compressor reliability may justify more operating margin and stronger evidence than a non-critical vent scrubber.

Do not convert that consequence directly into a generic “99.9%” request. A reviewable requirement still needs an inlet droplet and loading basis, operating cases and an agreed measurement method. Where those data do not exist, the honest route is conservative build-to-specification selection plus operating monitoring, not a fabricated guarantee.

What can go wrong

Liquid carryover can damage compressor internals, dilute lubricant, increase vibration or trip downstream protection. A demister also has to tolerate slugging, foaming, hydrocarbon condensate, corrosion and high gas density.

GPSA separator sizing methods are useful for first-pass review, but compressor suction service often deserves conservative K-factor selection and close attention to inlet distribution and liquid handling.

GPSA is an engineering data book, not a conformity standard. Its methods do not supply a universal project K value and do not replace compressor-vendor limits, vessel geometry or a liquid-surge review.

Mesh pad or vane pack?

A wire mesh pad can work well when droplets are fine, the service is clean and liquid loading is moderate. A vane pack can be more suitable where liquid load, fouling or slug tolerance is more important than compact fine mist capture.

If the scrubber has repeated carryover, do not simply buy a denser mesh. Higher density can raise pressure drop and flooding risk. Review inlet device condition, liquid level control, available disengagement space and actual gas flow.

Diagnose repeated carryover in order

1. Verify flow, pressure, temperature and gas composition against the current operating case.
2. Check level instruments, high-level events, drain restriction and evidence of liquid slugs.
3. Inspect the inlet device, demister elevation, outlet location and any direct flow path.
4. Look for wall bypass, displaced sections, sagging, collapsed grids or blocked drainage.
5. Compare pressure-drop history with carryover timing and deposit condition.
6. Only then decide whether media geometry, area or separator family should change.

A sudden event associated with high level is different from gradual deterioration as deposits build. The first points to bulk-liquid control; the second may indicate fouling or shrinking open area. Neither trend proves one cause without inspection.

Mechanical details are part of reliability

Specify support and hold-down loads for the agreed service, including flow direction and credible liquid accumulation. Section joints must not open during transport or operation. Wall sealing should prevent bypass without forcing an oversized pad into the vessel. The section map, match marks and installation orientation should be readable in the outage environment.

For offshore or remote installations, packaging should protect grids from bending and keep section identification attached through sea freight and storage. A correctly selected pad that arrives distorted is not a controlled replacement.

RFQ data that matters

Send actual gas flow, operating pressure and temperature, gas molecular weight or density, liquid density, liquid load, vessel ID, demister elevation, manway opening, inlet nozzle orientation, material preference and failure history.

Add minimum, normal, maximum continuous and upset cases; compressor type and relevant liquid-tolerance requirement; level and slug history; allowable pressure drop; existing inlet-device details; distance from high liquid level to the demister; and available distance from the demister to the gas outlet. State which values are measured, design data or estimates.

Define the QA scope in the RFQ so it is priced into the quotation and frozen in the PI: the material or batch traceability record you need, dimensional inspection against agreed measurement points and acceptance limits, pre-shipment photos, section marking and packing protection. Each shipment then carries those records as delivery evidence. EN 10204 Type 3.1 or Type 3.2 documentation, additional testing and third-party inspection are optional upgrades — request them before material procurement, production or inspection booking so traceability, cost and schedule are confirmed upfront. Compressor service is a poor place to accept undocumented internals.

For a build-to-drawing order, the supplier verifies conformance to the frozen construction; this does not imply compressor protection under an undisclosed duty. A performance-guaranteed route must separately freeze process inputs, acceptance criteria and responsibility for upstream controls.

Use the knock-out drum guide for the overall separation train and the inspection checklist before copying an old pad.

Engineering source and transfer boundary

Sulzer’s gas/liquid separation overview identifies compressor knock-out service and emphasizes the interaction of inlet distribution, separation media, liquid holdup and drainage. Treat any supplier’s published result as construction- and duty-specific; transfer it only when the offered design and operating basis are demonstrably equivalent.